mptcp: add sk_stop_timer_sync helper

[ Upstream commit 08b81d873126b413cda511b1ea1cbb0e99938bbd ]

This patch added a new helper sk_stop_timer_sync, it deactivates a timer
like sk_stop_timer, but waits for the handler to finish.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 584f3742890e ("net: add sock_init_data_uid()")
Signed-off-by: Sasha Levin <sashal@kernel.org>